The invention relates to an altimetry method and system for determining an elevation profile of a portion of the Earth surface. More precisely, the invention relates to a bistatic passive altimetry method, using opportunity signals such as GNSS signals and incorporating delay-doppler curvature compensation.
The invention can be applied, in particular, to the field of satellite altimetry for oceanography.
The possibility to measure with high accuracy the mesoscale ocean topography is of primary importance for oceanographers, meteorologists or climatologists in order to improve the understanding of ocean circulation, ocean bathymetry, eddies, tides and Earth climate models.
Over the last years, conventional radar altimeters have provided huge amount of data, allowing the observation of many ocean features. However, since they are based on observing the ocean on a small footprint along nadir-looking direction, classical altimeters such as TOPEX/Poseidon, Jason, or ESA RA and RA-2 are not able to provide high spatial-temporal sampling, absolutely necessary to map properly ocean mesoscale features, unless deployed in ad-hoc constellations.
The wide swath ocean altimeter has been envisaged as a potential solution to increase spatiotemporal sampling; however it is a very complex and costly solution. See W. J. Emery, D. G. Baldwin, D. K. Matthews, “Sampling the Mesoscale Ocean Surface Currents With Various Satellite Altimeter Configurations”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 42, No. 4, April 2004, 795.
In this context, since 1993 European Space Agency and later European industry have been working on the idea to make use of GNSS (Global navigation Satellite System) signals reflected from the ocean's surface in order to perform altimetry. The technique, designated as “PARIS” (Passive Reflectometry and Interferometry System), has been investigated and experimentally proven by ESA, NASA and many other space and non-space related research organizations. PARIS is a very wide swath altimeter, capable of reaching a swath of 1000 km or more, depending on orbital altitude, as it picks up ocean-reflected (and direct) signals from several GNSS satellites, up to 12 tracks when Galileo will be available.
For a detailed description of the PARIS technique, see:                M. Martin-Neira, “A Passive Reflectometry and Interferometry system (PARIS): Application to Ocean Altimetry”, ESA Journal, 1993;        U.S. Pat. No. 5,546,087; and        G. A. Hajj, C. Zuffada, “Theoretical Description of a Bistatic System for Ocean Altimetry Using the GPS Signal”, Radio Science, Vol38, No5, October 2003.        
Due to the global coverage and the bi- or multi-static nature of this technique, a low-Earth-orbiting PARIS instrument would allow high spatial-temporal sampling of the Earth surface. For these reasons PARIS has been identified as a very promising complementary technique with respect to conventional radar altimeters in order to address mesoscale altimetry or fast tsunami detection. The precision requirement in order to properly perform mesoscale altimetry is considered as 5 cm height precision over a spatial extent of 100 km at most. See P.Y. Le Traon and G. Dibarboure, G. Ruffini, E. Cardellach, “Mesoscale Ocean Altimetry Requirements and Impact of GPS-R measurements for Ocean Mesoscale Circulation Mapping, Technical Note Extract from the PARIS-BETA ESTEC/ESA Study” ESTEC, Dec. 2002.
However, a major disadvantage of this technique is that, being intrinsically passive, it is highly dependent on the characteristics of the available navigation signals. Indeed, transmitted signals power and bandwidth are the most important parameters driving the performance of an altimeter, either radar or PARIS based, either mono-static or bi-static. Currently transmitted navigation signals show significantly reduced power and bandwidth with respect to conventional radar altimeters. This implies poorer altimetry precision, accuracy and resolution per pulse. Several studies and results from airborne experimental data have predicted that a space based PARIS receiver exploiting GPS C/A code cannot meet ocean mesoscale altimetry requirements, even adopting maximum reasonable instrument dimensions.
On the other hand, the exploitation of a GPS P-code-like signal (which presents wider bandwidth, and, in turn, better performances) is on the limit of fulfilling the mesoscale requirements for a spatial resolution of 100 km, as shown by O. Germain and G. Ruffini in their paper “A revisit to the GNSS-R code range precision”, Proceedings of GNSS-R Workshop, 14-15 Jun. 2006, ESTEC.
The invention aims at improving the performances of the PARIS technique, i.e. its precision, accuracy and/or spatial resolution.